noun
plural -ups
[count] :a person or thing that can be used to replace or support another person or thing
a backup quarterback
[noncount] :help or support provided by additional people or things
The policeman called for backup [=called for other police to come and help him] because the suspect was carrying a gun.
[count] US :a situation in which the flow of something (such as traffic) becomes blocked
[noncount] :a musical part that is sung to support the main singer
[count] :a copy of information stored on a computer